A battery is a device that converts chemical energy into electrical energy.
A dry-cell flashlight battery consists of an electric cell, but larger batteries
are made up of a group of cells that are connected to act as a source of direct
electric at a given voltage. A cell consists of two dissimilar substances, a
positive electrode and a negative electrode, that conduct electricity, and a third substance,
an electrolyte, that acts chemically on the electrodes. A group of several such
cells connected together is called a battery. Batteries are widely used as a
source of direct current electrical energy in automobiles, boats, aircraft, portable
electric and electronic equipment, and lightning. A car battery is used mainly
to start the motor. Once the motor is running, an alternator supplies current
to recharge the battery. A few car battery companies include Diehard
and Trojan.

The formula for determining battery energy is:

E = P·t = V·I·t

Where

I = Current Expressed in amperesV = Electromotive force expressed in voltsP = Power expressed in wattst = Time expressed in hoursE = Energy in joule